1. Field of the Invention
This invention relates to an electrode for measuring an ion activity (concentration x activity coefficient) in a solution. More particularly, this invention relates to an electrode which uses a solid polymer membrane of an ion exchanger as an ion selective substance and is useful for the quantitative analysis of a specific anion or cation in the presence of various foreign anions or cations, and to an apparatus utilizing said electrode.
2. Description of the Prior Art
Currently available methods for the measurement of the activity of chloride ion in the blood through analysis for electrolytic content include the coulometric method, the ion electrode method and so forth. The coulometric method, when applied to a sample requires that one first remove corpuscular components from the whole blood and isolate the plasma and serum, dilute the isolated plasma and serum with a suitable diluent and subject the resultant dilution product to coulometric analysis. The ion electrode method involves inserting an ion electrode incorporating an ion sensitive substance into a given sample and measuring the electric potential which is generated between the ion sensitive substance and the sample. Various ion electrodes have been developed. These are typified by a glass electrode intended for the measurement of hydrogen ions (pH). If there were developed an ion selective electrode capable of directly measuring the concentration of chloride ion in the blood, the aforementioned procedures such as the centrifugation and dilution of the blood cells could be eliminated while still being able to determine the true concentration of chloride ion in the blood to be determined. A solid membrane electrode formed by using AgCl as its principal component is typical of ion electrodes sensitive to chloride ion. While this electrode has an advantage that it offers low electric resistance and permits easy measurement of electrode potential difference, it nevertheless has a disadvantage that, when used on a vital sample, it is possibly smeared with various components other than the objective ions such as, for example, proteins to a point where it exhibits degraded sensitivity and retarded response and loses in reproducibility of measurement and selectivity for chloride ion. A liquid membrane electrode has already been developed as an electrode sensitive to various anions. It is preprared by having an ion sensitive substance passed into and deposited fast in a porous carrier. Because of this particular construction, it is difficult to clean and suffers from a short service life. When this liquid membrane electrode is applied to a vital sample, it is smeared with the coexistent substances (such as, for example, proteins and blood cells) of the sample similarly to the solid membrane electrode of AgCl and, consequently, is degraded in sensitivity and response speed. (Refer to U.S. Pat. No. 3,429,785.) One version of the liquid membrane electrode has made use of an ion exchanger (Coetzee, C. J. & Freiser, H.: Anal. Chem. 1968, 40, 2071 & 1969, 41, 1128-1130). This version has not been immune from the problems common to all the liquid membrane electrodes. An electrode using a so-called "heterogeneous membrane", which has an ion carrier such as Valimomycin incorparated in a polymeric material, has been introduced (U.S. Pat. No. 3,753,887). In terms of performance, however, this electrode has never proved to excel the liquid membrane electrode mentioned above.